In recent years the progressive miniaturization of active elements in semiconductor integrated circuits has generated a critical need for microlithography technology that can achieve correspondingly finer resolution. This need has led to the development of projection microlithography in which, instead of using ultraviolet light as an energy beam, even shorter wavelengths are used such as soft X-rays having a wavelength in the range of approximately 10 to 15 nm. This new type of microlithography also is termed "EUV" (extreme ultraviolet) microlithography.
In the EUV microlithography wavelength range, the refractive indices of materials tend to be very close to 1. As a result, conventional refractive and reflective optical elements cannot be used. Rather, grazing incidence optical components or multilayer mirrors typically are used. A grazing incidence mirror exploits total reflection resulting from its refractive index being slightly less than 1, and a multilayer mirror exploits a multilayer film ("multilayer") that superimposes and phase-aligns weakly reflected light to produce a net high reflectance of the light.
A conventional EUV microlithography apparatus mainly comprises an X-ray source, an illumination-optical system, a mask, an imaging-optical system, a mask stage, and a wafer (substrate) stage. The apparatus "transfers" an image of a circuit pattern, as defined on the mask, to the wafer. So as to be imprinted with the image, the wafer is coated with an appropriate resist. The image is transferred to (projected onto) the resist by the imaging-optical system. The imaging-optical system typically comprises multiple multilayer mirrors.
The mask typically is a reflection-type mask as disclosed in, for example, Murakami,Hyomen Gijutsu (Surface Technology) 49:849, 1998; and Murakami et al., Jpn. J. Appl. Phys. 34:6696-6700, 1995. In such a mask, an absorber layer (comprising a substance highly absorptive to soft X-ray radiation) is formed, in a prescribed circuit pattern, on or in a multilayer that reflects soft X-rays.
As noted above, an EUV microlithography optical system typically comprises multiple multilayer mirrors and grazing-incidence mirrors. Thus, a soft X-ray beam is reflected multiple times as the beam passes through the optical system. Unfortunately, as the number of multilayer mirrors in the optical system increases, the full-width at half maximum (FWHM) of the reflectance spectrum of EUV light passing through the optical system correspondingly decreases.
Whenever there is a significant difference between the center wavelength of EUV light passing (by reflection) through an optical system consisting of multiple multilayer mirrors and the center wavelength of the EUV light reflected from the multilayer of the reflection mask, a decrease is observed in the combined reflectance of the optical system and the mask. As a result, the quantity of EUV light passing through the optical system and actually reaching the wafer is decreased undesirably.
If, over the plane of the reflection mask, there is a non-uniformity of the thickness period of the multilayer, then the reflectance of the reflection mask at the wavelength used in the microlithography apparatus will vary correspondingly according to position on the mask. This reflectance non-uniformity of the reflection mask is manifest as a non-uniformity in illumination of the wafer (located at an optically conjugate position relative to the reflection mask). As a result, exposure undesirably will vary at different locations on the wafer.
Also, whenever the exposure (i.e., total amount of light energy projected onto the resist on the wafer) exceeds a certain desired range, the linewidth of the circuit pattern transferred onto the wafer exhibits an excessive change that tends to degrade resolution.